This invention relates in general to electrical impedance balancing systems, and more particularly, to a transmission line impedance compensation system for dynamically balancing a reactive voltage component on the transmission line responsive to conditions on demand.
The invention is particularly applicable to a three-phase transmission line system which is operated at elevated voltage, current and other electromagnetic field parameters in order to provide efficient electric power transmission. The transmission of electric power ranges over vast easement corridors which attract the likelihood of and are subjected to prolific fault conditions at various times. These include events such as a distribution of lightning strikes, as well as, natural and unavoidable fault bridging events, which occur in or out of operational high power consumption periods. These problematic events cause impedance changes in the respective power transmission lines which may last either momentarily or for prolonged periods of time until cleared by burn-through and physical removal.
Electrical impedance balancing circuit arrangements for transmission line systems have heretofore comprised serially connected capacitors (often referred to as series connected). A classical series circuit transmission line connection is defined with the relevant actively compensating capacitors experiencing the same current. A thyristor-switched capacitor arrangement in FIG. 1 is illustrative of the prior art. Transmission line components are rated in a voltage class, or series of classes, typically within an overall range from about 240 Kilo-Volts (KV) to about 750 KV, for the high voltage potential of the transmission line operational characteristic for this type of circuit. One of the problems has been that the capacitor banks, operated at those relatively high voltage potentials, must be each controlled by a similarly voltage rated bypass switch or valve. These switches are comprised of a circuit chain of reverse parallel connected thyristors which are operated at the high voltage potential of the transmission line.
The above-described arrangement provides relatively fast transmission line impedance control to change the degree of series impedance compensation for the transmission line. The bypass switches constituted by the thyristor pairs are each connected to a data channel that must be provided with voltage breakdown isolation between the relatively high transmission line voltage potential and a control circuit which is based at ground voltage or a relatively low reference voltage potential, and this is well below the line voltage potential magnitude. Aside from the considerable time and cost of engineering and other skills needed for the development, manufacture, installation and maintenance of the high voltage hardware suitable for the task, a capacitor switching circuit arrangement does at least partially fulfill, by cancelling a part of the inductive reactive impedance of the transmission line, the objective of increasing the power transmitting capability of the transmission line.
One of the principal problems with transmission line electrical impedance balancing circuits of this type is that the electrical resonance effect, which can occur when the inductive reactance of the transmission line and the switched-in series compensating capacitive reactance coincide with a specific frequency. This frequency is the difference between the line frequency and one of the mechanical resonant frequencies of the rotating elements of the combined generator and turbine system, and it can cause "subsynchronous resonance". This type of resonance is the oscillation that results in an oscillatory machine torque that can cause an increasing or growing oscillation of the shaft speed about a steady-state frequency. A dangerous mode of operation is the result, so much so as to cause damage to the turbine shaft and, consequentially worse, the catastrophic breakage of said shaft. Should this breakage occur, the downtime for repair or replacement of essential system components is a monumental financial burden further considering the irretrievable loss of operating revenue, as well as the jeopardy of accumulated good will from power system users.
Another problem with electrical impedance balancing circuits of this type is that there may be a sudden sharp increase or decrease in the transmitted electric power. This is a problem because electrical transmission systems are frequently subjected to disturbances over a wide spectrum of severity or types. Some examples given above are lightning strikes and line faults, but other disturbances occur when large loads are being switched on and off. Other types of disturbances include the insertion or deletion of some power generators, and likewise, there is a problem when some of the parallel transmission lines are being switched on and off. A problem arises if the disturbance results in a sudden sharp increase or decrease in the transmitted electrical power, and this cannot be immediately matched by a change in the mechanical output power of the turbines which drive the power generators. Consequently, the generators are forced to accelerate or decelerate with the presence of oscillations, thereby affecting rotational speed and angular shaft position. This alters the amount of power transmitted even so much as to cause a temporary power shutdown in order to prevent an even more serious loss of equipment.
It is known to provide a voltage-source inverter with a fixed voltage DC link in high performance motor drive applications such as in U.S. Pat. No. 4,697,131 entitled, "Voltage Source Inverter and Variable Frequency, Constant Voltage AC Motor Drive Embodying The Same", which patent is assigned to the same assignee as the present application. This U.S. patent is hereby incorporated by reference into the present application and will be hereinafter referred to as the incorporated U.S. Patent. It teaches a regeneration mode of operation in which a motor drive is reliant on diode means in preparing for recovery after a controlled power switch has interrupted the current path. The diode means also is used to bypass the controlled power switch in the forward power mode of operation of the motor drive.
The above referred type of voltage-source inverter is used in an adaptive situation to maximize the amount of inductance inserted in a variable voltage DC-link, according to U.S. Pat. No. 4,805,082 entitled "Regenerative Two-Quadrant Converter", which is assigned to the same assignee as the present invention. Maximized inductive insertion is done by using a GTO device or a transistor to interrupt the main current so as to reduce the harmonic content thereof. It also is used to limit the rate of rise of a fault current in the event of a "shoot-through" among the thyristors of the converter bridge while in the regenerative mode. An additional static switch is thus provided for a separate bypass for the inductance energy through which to "freewheel" so as to help build up and thereby minimize the duration of a zero-current condition in the bridge at the moment of commutation.
An object of the present invention is to utilize some of the considerations from the above described voltage-source inverter and converter circuits and to apply them independently in a controllable series compensation circuit for dynamically balancing the reactive impedance in a transmission line system. This is done to provide static and dynamic stability while producing enhanced efficiency of power transmission.